Abstract
Factors affecting the appearance of 7~8 wt.% yttria stabilized zirconia thermal barrier coatings were investigated. The coating appearance can be separated into two aspects, darkening and coloring. Coating darkening is related to the retaining of off-stoichiometric zirconium oxide (ZrO2-x) or yttrium oxide (Y2O3-x). These off-stoichiometric oxides are stable at temperatures between 2300 °C and 2700 °C. Rapid cooling of these oxides leads to the retaining of them in the as-sprayed coatings. A heat treatment in air at temperatures higher than 500 °C transforms these off-stoichiometric oxides to stoichiometric oxides and therefore eliminates the coating darkening effect. Coloring of TBCs is attributed to the existence of impurity oxides. Coatings made from the ultra high purity powder, Z204NS, do not exhibit coloring phenomenon before or after heat treatment. As-sprayed coatings made from premium purity (M204NS) or regular purity (A204NS-1) powders exhibit metastable coloring effect. A 1-hour heat treatment at 500 °C can eliminate the metastable colors and turn the coating color back to the color of the starting powder.
